Add some padding in WINPROC_wrapper

[Mike Hearn]

> It may work for one application, but it doesn't look like a propper fix. 
>  I'd suggest there's probably something else going on that's causing 
> stack corruption.

Probably it was declared with the wrong calling convention. Windows 
deals with this transparently but perhaps more transparently than us :)

So the question is: what calling conventions could cause the observed 
behaviour?